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D. GRIMES INVERSE DUPLEX VACUUM TUBE CIRCUIT Filed Dec. 1 1923 5Sheets-Sheet 2 ATTORNEYS Nov. 25,1924. 1,517,058

D. GRIMES INVERSE- DUPLEX VACUUM TUBE CIRCUIT Filed Dec. 1, 1923 5Sheets-Sheet TI M I I I I Q INVEI'QTOR Jami firzmes A'ITQRNEYS Nov. 25,1 924.

D. GRIMES INVERSE DUPLEX VACUUM TUBE CIRCUIT Filed Dec. 1. 1923 5Sheets-Sheet 4 lvvvvvv INVENTOR javzk? rzmes BY D E ./%M wW ATTORNEYS YNov. 25, 1924- D. GRIMES INVERSE DUPLEX VACUUM TUBE CIRCUIT 1 1923 5Sheets-Sheet 5 Filed Dpc.

INVENTOR Jaw/ a fi'rl'm I BY Z 3 z nzr ATTORNEYS Patented Nov-25,1924

UNITED STATES PATENT OFFICE.

DAVID GRIMES, OF GRASMERE, NEW YORK, ASSIGNOR TO GBIMES RADIO ENGINEER-ING 00., INCORPORATED, OF GRASMERE, NEW YQRK, A CORPORATION OF NEW YORK.

INVERS -DUPLEX VACUUM-TUBE CIRCUIT.

Application filed December To all whom it may concern:

Be it known that I, DAVID Gnnvrns, a citizen of the United States,residing at Grasmere, in the county of Richmond, State of New York, haveinvented certain new and useful Improvements in Inverse-DuplexVacuum-Tube Circuits; and I do hereby declare the following to be afull, clear, and exact description of the invention, such as will enableothers skilled in the art to which it appertains to make and use thesame.

This invention relates to thermionic amplifiers, and is directed, moreparticularly, to improvements in multi-stage amplifiers of the inverseduplex type.

In my copending application Serial No. 589,108, there is described andclaimed a vacuum tube amplification and reception system wherein theamplifier tubes are arranged to effect both radio frequency and audiofrequency amplification simultaneous 1y, but in which the order ofamplification differs from the previously known reflex method in thatthe audio frequency amplification is accomplished in the inverse orderas compared with the order or sequence in which the tubes successivelyfunction to effect radio frequency amplification. Systems of thisv typeare now identified by the term inverse duplex.

Prior to the introduction upon the market of vacuum tube amplifiers ofespecially high efficiency, which have only recently been produced,such, for example, as the vacuum tube amplifier known under the tradesymbol UV QOGA, no difiiculties had been experienced with inverse duplexamplifiers arising from audio frequency oscillations being sustained,which condition, as is well known, results in continuous howling.Following the introduction of the more efiicient amplifier tubes,however, it was found that sustained audio frequency oscillations weresometimes produced by the use of these tubes with. inverse duplexsystems.

To remedy this undesirable condition is the object of the presentinvention.

In inverse duplex vacuum tube systems it is the practice to connect theoutput circuit of one vacuum tube amplifier with the input circuit ofthe succeeding vacuum tube amplifier through the medium of a radiofrequency transformer. After rectification of the amplified radiofrequency signaling our- 1, 1923. Serial No. 677,955.

rents, the resultant audio frequency currents are transmitted throughthe medium of an. audio frequency transformer to the input circuit ofthe. last radio frequency amplifier tube wherein audio frequencyamplification is effected. The amplified audio frequency currents arethen transmitted from the output circuit of the last-mentioned tubethrough the medium of a second audio frequency transformer to the inputcircuit of the preceding radio frequency amplifier tube, wherein furtheraudio frequency amplification is effected, thus resulting in theproduction of audio frequency signaling currents in the output circuitof the last mentioned radio frequency amplifier tube. If it is possiblefor sufficient audio frequency currents to be transmitted from theoutput circuit of the last-mentioned amplifier tube through the capacitycoupling of the radio frequency transformer to effect considerable gridpotential variation in the succeeding amplifier tube, sufficient furtheramplification of the audio frequency currents may be produced toovercome the attenuation of the capacity coupling path through the radiofrequency transformer and thus cause sustained audio frequencyoscillations by reason of the feedback between the two amplifier tubes.

lVhere it is not nacticable to sufficiently reduce the capacity couplingbetween the primary and secondary windings of the radio frequencytransformer on account of other controlling factors, the ren'iedy, inaccordance with this invention, lies in a nunr bcr of alternativecircuit arrangements whereby the amplified audio frequency current inthe output circuit of a preceding tube is prevented from effecting thepotential of the grid of a succeeding tube sufliciently to overcome theattenuation of the capacity coupling path of the transformer. In someinstances the audio frequency current is shunted away from the primarywinding of the radio frequency transformer, while in other instances itis permitted to pass through the capacity coupling path of the radiofrequency transformer, but is prevented from materially affecting thepotential of the grid of the succeeding amplifier tube.

In addition to the circuit arrangements provided by this invention forpreventing the maintenance" of continuous audio fire quency oscillationsresulting from the inherent capacity coupling between the windings ofthe radio frequency transformer interconnecting adjacent amplifiertubes, a further feature of the invention resides in the provision ofcircuit connections whereby audio frequency currents which may betransmitted through the inherent capacity coupling of the radiofrequency transformer interconnecting the last stage of radio frequencyamplification with the detector circuit are rendered ineffective tocause the maintenance of continuous audio frequency oscillations. Thisfeature consists in the provision of a low impedance path from thedetector circuit to the cathode of the amplitier, this path being inshunt to the high impedance path to cathode by way of the audiofrequency transformer in the detector circuit and the grid of theamplifier tube. By passing these leakage currents directly to cathodeinstead of to cathode via grid, the potential of the grid is notaffected and there is accordingly little or no tendency for the circuitto oscillate.

A number of alternative arrangements which will accomplish the object inview are illustrated in the acconu'ianying drawings and described indetail hereinafter. These several circuit arrangements are shown only byway of example, and are not intended to be exhaustive of all thepossible alternative arrangements within the scope of the invention.

Referring to the drawings,

Fig. 1 is a circuit diagram of a two-tube inverse duplex amplifier witha crystal de tector arranged in accordance with the previous practice(that is not embodying this invention) which is provided for the purposeof comparison in order to render a clearer description of the invention;

Fig. 2 is a circuit diagram similar to Fig. 1 but modified in accordancewith this invention;

Fig. 3 is a circuit diagram also similar in general to Figs. 1 and 9.,but illustrating an alternative modification;

Fig. l illustrates still another alternative arrangement and is likewisebased on the circuit of Fig. 1;

Fig. 5 is a further alternative circuit arrangement; and

6 is a circuit diagram of an inverse duplex vacuum tube amplifier andreceiving circuit including a single-tube power amplifier.

The circuit shown in Fig. 1 does not, as previously stated, embody thepresent invention. but is shown more particularly for the purpose ofcomparison, that is, for the purpose of illustrating the conditionswhich, it has been found, sometimes result in sustained audio frequencyoscillations when vacuum tube amplifiers of high efiiciency areemployed. The circuit now under discussion includes two stages of radiofrequency amplification, two stages of audio frequency amplification,and a crystal detector. Both types of amplification are ell'ected by thesame tubes simultaneously in accordance with the inverse duplexprinciple. The coil 1 may be a loop antenna, or it may be the secondaryof a rariocoiqilcr the primary of which may be connected in an antennacircuit of the usual type. It may be assumed in the presentiust-ancethat the coil l represents a loop. Across 1 terminals of the loop l. isconnccted a variable condenser 2 forming an oscillatm y circuit with theloop which may be tuned to the frequency of the desired incomingsignals. The loop 1 is preferably provided with a plurality of contactsfor cooperating with the variable contact 3, which is connected throughthe conductor t to the grid of the three-electrode vacuum tubean'lplifier 6. The intensity of signal. strength applied to the grid 5may be \aried by adjusting the moviible contact 3 to one or the otherloop contacts. This may be done without appreciably affecting the tuningof the loop circuit. redu g the number of loop turns between the gridand filamentary cathode 7, the effective strength of signals from localstations may be reduced so as to prevent overloading the amplifiertubes. For receiving signals from more distantly located stations, alarger number of turns of the loop would ordinarily be connected intocircuit bctween the grid 55 and filament 7, thus in creasing the effectof the incoming signals on the grid. The tube 6 has a radio fre quencyoutput circuit including plate 8, primary winding 9 of the radiofrequency transformer ll, condenser 10, which may be of relatively smallcapacity, and filament 7. The circuit just defined will pass radiofrequency oscillations, but, on account of the sinall capacity ofcondenser 10, will pass little or no audio frequency current. Theamplified radio frequency currents are induced from the primary 9 to thesecondary 12 of the radio frequei cy transformer ll and thus impressedacross the grid 14 and filament 153 of tube 13. A condenser 16 isincluded in the grid-filament circuit of the latter tube. This condenserwill pass the raoio frequency oscillations while blocking audiofrequency oscillations. The radio frequency oscillations are furtheramplified by the tube 13 and passed through the radio fre'qiiencytransforn'ier 17 to a circuit including the secondary winding 18 of thelast-mentioned transformer, crystal detector l9, primary winding 20 ofthe audio frequency transformer 21 and condenser in parallel with thelatter wind ing.

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harness flhc amplified radio frequency oscillations are rectified in theusual manner by the crystal detector 19 and converted into the form ofaudio frequency signaling currents. The audio frequency currents areinduced from the primary 20 into the secondary 23 of transformer 21, theterminals of which are connected respectively to the grid l-l andfilament 13 of amplifier tube 13. 'lhe latter tube thus functions toeffect the first stage of audio frequency amplification. The audiofrequency output circuit of the tube 13 includes plate 2%, primarywinding oi the radio frequency transformer 17, primary winding 26 of thesecond audio frequency transformer 2'7, battery 28 and filament 15. Thevarious connections may be readily traced. The amplified audio frequencysignaling currents are induced from the primary 26 into the seconoary529 of transformer 27. The terminals of the secondary winding 29 areconnected respectively to the grid 5 and filament 7 of the vacuum tube(3 through a circuit which includes a portion of the turns of the loopl. Further audio frequency amplification is effected by the tube 6, theaudio frecuiency output circuit of which includes plate 8 primarywinding 9, telephone receivers 230, battery 28 and filament 7.

Between the primary and secondary windings of each of the radiofrequency transformers 11 and 17 there exists an in herent capacitycoupling which is illustrated in dotted lines and referred to byreference numerals 31 and 32.

Taking note of the fact that the audio frequency output circuit of thetube 13 is associated through the medium of the audio frequencytransformer 27 with the input circuit of the amplifier tube 6, itfollows that if the capacity coupling 3., and amplification factor oftube 6 are relatively high, a sufiicient amount of audio frequencyenergy will pass through the capacity coupling 31 to appreciably affectthe poten tial of grid 14 and thus maintain con tinuous audio frequencyoscillations. One remedy for the condition described is to employ vacuumtubes having a relatively low amplification facton that is, of lowefficiency; another remedy may be to separate the primary and secondarywindings of the radio frequency transformer to such an extent as togreatly reduce the capacity coupling therebetween. Each of the foregoingremedies, howeven results in lowered elliciency.

According to the present inventiom a number of different circuitarrangements have been found whereby this objectionable condition can beovercome without sacrificing the increased efficiency insulting; fromthe employment of close coupling and high amplification tubes. i i hilethe several alteri'iative circuit arrangements illustrated appear todiffer radically in form, there is an underlying principle common to allof them.

Still referring to Fig. 1, let it be assumed that the grid 5 is for aninstant rendered negative by the amplified audio frequency potentialimpressed thereon. This will result in a stoppage 0 reduction of flow ofspace current within the tube The in-- ductive discharge from thetelephones 30 in following the path of least resistance will passthrough the capacity coupling 31.. thence tl'irough grid '14. tofilament 15 and battery 28, thereby coi'npleting; a discharge circuitforthe phones 30. lhe action just enumerated results, of course, in achange of potential on the grid la, the further re sult of which is an.amplified reproduction of the audio frequency current variation in theoutput circuit of tube 13. This audio frequency current variation isthen repeated back to the input circuit of tube 6 through the medium oftransformer 27. The condition is therefore a regenerative one, resultingin the maintenance of oscillations the frequency of which is inaccordance with the constants of the circuit.

A similar condition exists between the amplifier 13 and the detectorcircuit, arising from the inherent capacity coupling 32 of transformer17. Any substantial audio frequency leakage through the capacity pathwill tend to flow to cathode by way of grid 14 and the capacity pathbetween the windings of transformer 21.. lhis condition is overcome ineach of the several alternative circuit arrangements in the mannerhereinafter described.

In Fig. 2 the phones 30 are connected between the plate 8 and theprimary winding 9 and are shunted by a sma l condenser 33. In thiscircuit arrangement there is little or no tendency for currentdischarges from the telephone receivers 30 to seek a path to filamentthrough prid 14 because of the fact that there is a very much easierpath to filament through the battery 28. Of course. when the resistanceof the space path through tube 6 of Fig. 2 is increased by reason ofincreased negative potential on grid 5, there is some tendency for thecurrent from battery 28 to follow the. capacity coupling path 31 to gridi l and thence to filament, but the potential of battery 28 is small ascompared with the instantaneous induced potential arisingfrom the suddencollapse of the n'iagnetic field of the tcle iihone receivers 30 so thatthere is a vast difference in the resultant effect on the potential ofgrid 14;. This ar ane'enient does actually eliminate the conditiondescribed which resulted in the maintenance of continuous audiofrequency oscillations and permits the use of high efficiency vacuumtubes and. radio frequency transformers haying :cnsiderable capacitycoupling.

in the present circuit, as well as .111 each of those to be describedhereinafter, the detector circuit is tied to the cathode 15 by conductorit) which provides a shunt path for audio frequency leakage through thecapae ty coupling 32. By careful examination of the circuit it will beseen that the conductor 4.6 parallels a somewhat tortuous circuit whichmay be traced as follows: from the output circuit of tube 13 tl'iroughti. 0 ca orudty coupling 32, one or both sides of the detector circuitto primary winding 20, which for the present consideration functions asa condenser plate in cooperation with the secondary winding 23functioning as the other plate, thence through grid 14; and the spacepath to cathode 15. Any sul'istantial current flowing through the pathjusttraced would, of course, affect the potential of grid 14. But sincethe current which would otherwise tend to flow in that path is shunteddirectly to cathode tl'irough the conductor 16, its effect is obviated.

1n circuits like that of Fig. 2, radio frequency transformers havinginherent capacity coupling between the windings of .001 microfarad havebeen employed with good results. So far as is known, any good commercialradio frequency or audio frequency transformers may be employed withthis circuit as well as with the circuits to be hereinafter described.The voltage of the plate battery 28 may range from to 130 volts withgoo-d results. it is thought that still higher voltages may be employedwithout any detrimental effect when using vacuum tubes designed forhigher plate. voltages.

In the circuits of Fig. 3 the audio frequency output in the platecircuit of vacuum tube 6 is substantially prevented from reaching theprimary winding 9 of the transformer 11 by means of a small blockingcondenser 3 1. The capacity of this condenser may be of the order of.001 microfarad. The audio frequency output circuit of tube 6, whichincludes telephone receivers 30, is connected in parallel with theprimary winding 9. ln order to prevent the transmission of radiofrequency currents through the audio frequency path which mightotherwise occur due to the capacity of the telephone cord, a radiofrequency choke coil 35 is inserted in the audio frequency outputcircuit as shown. The inductance of coil 35 may range from 25 to 200millihenrics. These values are given only by way of example, and are notto be regarded as limiting. It is thought to be obvious without furtherdiscussion that very little, if any, audio frearness quency current canreach the primary winding 0 through condenser 34;, and therefore littleor none can pass through the inherent capacity coupling of thetransformer 11 to affect the grid 14: of the succeeding amplifier tubes.Any audio frequency current which may pass the condenser 34 has a lowimpcdance path to filament through conductor 4;? and therefore has verylittle tendency to pass the coupling capacity 31. The circuit of Fig. 3may be easily understood by comparison with that of Fig. 1, from whichit differs only as already noted.

The circuit of Fig. f is soinewl'iat analogous in its operation to thatof In this case, however, no provision is made against the transferenceof audio frequency energy through the inherent capacity cou pling of theradio frequency transformer 11. ltn order to prevent such energy beingimpressed on the grid 14:, a small condenser 36 of capacity .0005microfarad approximately, is inserted in the grid-filament circuit oftube 13 as shown. The audio frequency input to grid 14 is connectedbetween the grid and the condenser 36, a radio frequency choke 37 beingpreferably included in this audio frequency input circuit. In someinstances it may be possible to omit the choke coil 37. Whether or notthis coil may be omitted depends very largely upon the pres ence orabsence of radio frequency shunts across the terminals of the secondarywinding This fact applies equally to the circuit of Fig. 3. In allinstances where it is found that the audio frequency input circuit tothe tube 13 provides a substantial path for the radio frequencycurrents, it is desirable, but not necessarily essential, to insert thecoil 37. The inductance of coil 37 mayrange from 25 to 200 millihenries.These values, however, are merely given by way of exam ple, and are notto be regarded as limiting.

1n the circuit of Fig. 5 no provision is made to guard against thetransference of audio frequency energy through the inherent capacitycoupling of the radio frequency transformer 11. In this circuit thesecondary winding 23 of transformer 21 is connected in series with thesecondary winding 12 of the radio frequency transformer 11, and isinterposed between the grid 14 and winding 12, there being a smallcondenser 38 shunted across the terminals of the winding 23. lVith thisarrangement the winding 12 is connected directly to the filamentcircuit. The path to filament for any audio frequency energy which maybe transferred through the capacity coupling of transformer 11 by way ofthe grid 14: is of such enormously greater impedance, on account of theinclusion of Winding 23, than the direct path to filament from thewinding 12 via conductors 39, 40 and 4-1, that the potential of grid 14is not likely to be materially afiected by such energy. The capacity ofcondenser 08 may be of the order of .0005 microfarad with good results.

The circuit of Fig. 6 is similar, in general, to that of Fig. 3, butincludes an additional power amplifier tube 42. The primary winding 43of transformer 4:4: is inserted in the audio frequency output circuit oftube 6. The secondary winding a5 is connected in the grid-filamentcircuit of tube 42. The telephone receivers 30 are connected in theplate circuit of tube 42. In so far as the present invention isconcerned, the disclosure of Fig. 6 is identical with that of Fig. Thiscircuit is shown, however, in order to make it clear that a poweramplifier can be combined with the inverse duplex circuit described inthe other figures.

In all of the several circuits shown, the telephone receivers 30 may bereplaced by a loud speaking instrument.

In each of the circuits shown the crystal detector circuit can bereplaced by a vacuum tube detector circuit without detriment.

I claim:

1. In an amplifier including at least two electron discharge deviceseach comprising anode, cathode and control electrode, wherein saiddevices both operate simultaneously for effecting both radio frequencyamplifica-- tion and audio frequency amplification, the method ofpreventing the maintenance of audio frequency continuous oscillationswhich consists in reducing or preventing the flow of audio frequencycurrent from a current source in the output circuit of one of saiddevices to the cathode of the last-mentioned device through the mediumof the control electrode and cathode of the other of said devices,thereby reducing or preventing audio frequency potential difierencevariations between the last-mentioned control electrode and cathodearising as a result of audio frequency current variations in said outputcircuit.

2. In an inverse duplex amplification system, two electron dischargeamplifiers each comprising an anode, a cathode and a control electrode,said amplifiers being operatively associated so as to consecutivelyeffect radio frequency amplification in a prede termined order and audiofrequency amplification in the inverse order, a radio frequency outputcircuit for one of said amplifiers and a radio frequency input circuitfor the other of said amplifiers, said radio frequency circuits beingassociated to effect radio frequency transmission from the first radiofrequency amplifier to the second radio frequency amplifier, an audiofrequency input circuit for said first radio frequency am plifier and anaudio frequency input circuit for said second radio frequency amplifier,means for impressing audio frequency currents on the audio frequencyinput circuit of said second radio frequency amplifier, the audiofrequency currents being amplified by the second radio frequencyamplifier, an audio frequency output circuit for said second radiofrequency amplifier, said lastmentioned audio frequency output circuitbeing operatively associated with said audio frequency input circuit ofsaid first-mentioned radio frequency amplifier for effecting thetransmission of amplified audio frequency currents from the output ofsaid second radio frequency amplifier to the input of said first radiofrequency amplifier, and means for substantially reducing audiofrequency potential difference variations be tween the cathode andcontrol electrode of said second radio frequency amplifier arising fromaudio frequency current variations in the output of said first radiofrequency amplifier.

3. An inverse duplex amplification system including two electrondischarge amplifiers each having an anode, a cathode and a grid, a radiofrequency transformer comprising a primary and a secondary winding forinductively interconnecting th output side of one of said amplifiers tothe input side of the other of said amplifiers for the transmission ofradio frequency signaling current therebetween, said primary andsecondary windings having inherent capacity coupling capable ofproviding a path for audio frequency signaling currents of sufficientenergy to effectively vary the potential of the grid of thelast-mentioned amplifier if permitted to flow to said grid, as a resultof which condition undesirable audio frequency oscillations might besustained, and circuit means adapted to prevent suflicient audiofrequency currents flowing through the path including the said capacitycoupling and said lastmentioned grid to the cathode of said firstamplifier to effect sufiicient audio frequency amplification to sustainaudio frequency continuous oscillations.

4:. An inverse duplex amplification system comprising at least twoelectron discharge amplifiers connected in cascad and adapted to effectradio frequency and audio frequency multi-stage amplificationsimultaneously, circuit connections for transmitting radio frequencysignaling current through successive amplifiers in a certain order,circuit connections for transmitting audio frequency signaling currentthrough said amplifier successively in the inverse order with respect tothe order of radio frequency amplification, and circuit connectionsarranged to prevent the effective transmission of andio frequencysignaling currents from the output side of one amplifier to the inputside of a successive amplifier in the order predetermined for radiofrequency amplification.

5. An inverse duplex receiving system comprising a plurality ofthree-electrode electron discharge amplifiers, each of said amplifierscomprising an anode, a cathode and a grid, a detector clrcuit operableto convert modulated radio frequency currents into audio frequencycurrents, circuit connections for operatively associating saidamplifiers and said detector circuit in such manner that said amplifiersare operable to effect multi-stage radio frequency amplification in apredetermined order and further operable toeffect multi-stage audiofrequency amplification in the inverse order as compared with theaforementioned predetermined order, the arrangement being such that thefully amplified radio frequency current is converted by said detectorcircuit into audio frequency current, the output side of at least one ofsaid amplifiers being associated with the input side of the succeedingamplifier through the medium of a radio frequency transformer theprimary winding of which is interposed in the output circuit of thefirst-mentioned amplifier and the secondary Winding of which isinterposed in the input circuit of the second-mentioned amplifier, asignal indicating device interposed in the output circuit of saidfirstanentioned amplifier between the anode thereof and the primaryWinding of said radio frequency transformer, and a radio frequency pathin shunt to said indicating device.

6. In a. signal receiving system, a threeelectrode electron dischargeamplifier comprising anode, cathode and grid, a radio fre quency inputcircuit for said amplifier including said grid and said. cathode, aradio frequency output circuit for said amplifier including said anodeand said cathode, a radio frequency transformer comprising a primaryWinding and a secondary Winding, there being inherent capacity couplingbe tween said windings, said primary Winding being connected in theradio frequency output circuit of said amplifier, detector circuitincluding the secondary winding of said radio frequency transformer, anaudio fre-- quency transformer comprising a primary winding and asecondary winding, the primary winding of said audio frequencytransformer being connected in said detector circuit, an audio frequencyinput circuit for said amplifier including the secondary winding of saidaudio frequency transformer together with said grid and said cathode,the arrangement being" such that modulated radio frequency currents areamplified by said amplifier and transmitted through said radio frequencytransformer to said detector circuit, said detector circuit beingoperable to convert said radio frequency currents into audio frequencycurrents which are trans mitted through said audio frequency transformerto the audio frequency input circuit of said amplifier, and a conductingpath of low impedance from said detector circuit to said cathode, saidpath being operable to form a short circuit for audio frequency currentswhich may pass through the inherent capacity coupling of said radiofrequency transformer, thereby preventing the maintenance of continuousaudio frequency oscillations in said system.

.ln testimony whereof I affix my signature.

DAVID GR IMFJS.

